WO2009125101A1

WO2009125101A1 - Improved process for producing hydrocyanic acid

Info

Publication number: WO2009125101A1
Application number: PCT/FR2009/050472
Authority: WO
Inventors: Enzo Formentin; Catherine Schafer
Original assignee: Arkema France
Priority date: 2008-03-20
Filing date: 2009-03-20
Publication date: 2009-10-15
Also published as: ATE518808T1; CN101977844B; FR2928910B1; ES2369967T3; US20110033362A1; EP2260002B1; EP2260002A1; AU2009235293A1; FR2928911A1; CN101977844A; FR2928910A1; MY150490A; FR2928911B1; JP2011515316A; US8574530B2; JP2015143183A; AU2009235293B2

Abstract

The present invention relates to an improved process for producing hydrocyanic acid by reaction of ammonia with methane in which a small amount of at least one sulphur-containing compound corresponding to the general formula R – S – (S)_x – R’ is added, in which R and R’, which are identical or different, represent a linear or branched alkyl or alkenyl radical containing from 1 to 5 carbon atoms, and x is a number ranging from 1 to 5, to the reactive gas mixture before it passes over the catalyst. The process according to the invention makes it possible to obtain improved yields of HCN. Another subject of the invention relates to the use of the resulting product for producing methionine, acetone cyanohydrin, adiponitrile or sodium cyanide.

Description

IMPROVED PROCESS FOR THE PRODUCTION OF CYANHYDRIC ACID

The present invention relates to the manufacture of hydrocyanic acid and more particularly to an improved process for producing hydrocyanic acid by reaction of ammonia on methane in which a sulfur compound belonging to the family of polysulfides is used. such as dimethyl disulphide.

Hydrogen cyanide HCN finds many applications as a reagent in various synthetic processes or as a synthesis intermediate. It is in particular a key reagent for the preparation of acetone cyanohydrin, synthesis intermediate for the production of methyl methacrylate MMA, monomer base thermoplastic polymers such as the

PMMA (Altuglas ^® , Plexiglas ^® ). Hydrocyanic acid is also used in the synthesis of methionine, or to manufacture the synthetic intermediate adiponitrile of polyamide 6.6 (Nylon ^® ) and many chelating agents. Sodium cyanide, a derivative of HCN, also has many applications in the chemical industry.

The industrial production of HCN hydrocyanic acid today is based mainly on the Andrussow process dating back to the 1930s. This process consists of reacting methane or natural gas with ammonia in the presence of air and possibly oxygen on a catalyst composed of rhodium-plated platinum webs. Since the reaction CH ₄ + NH ₃ - → HCN + 3H ₂ (1) is endothermic, the addition of air makes it possible, thanks to the combustion of a part of the hydrogen produced and the excess of methane, to have a globally exothermic system and thus maintain the synthesis reaction without external energy input.

The reaction, known as ammoxidation, is as follows:

CH ₄ + NH ₃ + 3/2 O ₂ -> HCN + 3H ₂ O + heat (2) The process is based on reactions (1) and (2). Due to the extreme sensitivity of the catalyst to poisoning by certain impurities (iron, sulfur ...) the quality of raw materials must be the best possible. Methane of greater purity than 90% containing the minimum of higher hydrocarbons (ethane and especially propane) and sulfur-free. The ammonia is filtered and evaporated and preferably does not contain oils or iron. The air is dusted by washing with water before being compressed. The three reagents (CH ₄ , NH ₃ , air) are mixed in precise stoichiometric proportions. The resulting gas stream is, after being filtered, introduced into the reactor. This consists of rhodium-plated platinum webs placed on a support and a quenching boiler for cooling the gases immediately after contact with the catalyst. The initiation of the reaction is carried out by means of an electrical resistance system which lights the canvases. Once this ignition is achieved, the overall exothermicity of the reaction maintains the canvases at 1050 - 1150 ^° C.

The kinetics is very fast with a contact time close to a few milliseconds or tenths of a millisecond, and a gas velocity of the order of a few meters per second. The proportion of each reagent is optimized so as to obtain a maximum yield and avoid the flammable zone of the reaction mixture.

The reaction generally reaches a yield of 60 to 70%, expressed as the number of moles of hydrocyanic acid produced over the number of moles of ammonia introduced, the conversion of methane being almost quantitative.

The selectivity to hydrocyanic acid is generally 80 to 90% (number of moles of HCN produced on the number of moles of NH ₃ reacted).

Another process for producing HCN, the Degussa process, is based on the aforementioned reaction (1), in the absence of oxygen or air, at a temperature of the order of 1300 ^° C. The reaction is carried out then in sintered alumina tubes internally coated with platinum, the bundle of tubes being heated with gas inside an oven.

In these processes, a part of the ammonia introduced to react on the methane, does not participate in obtaining HCN, and either decomposes into nitrogen and hydrogen according to the reaction: 2 NH ₃ - * N ₂ + 3 H ₂ or remain inert (by-pass). The first purification step then consists of neutralizing the unconverted ammonia with sulfuric acid. The ammonium sulphate solution thus generated is stripped with steam to rid it of traces of HCN still present. HCN contained in the gases freed of NH ₃ is then absorbed in water. At the head of this absorption column are mostly only inert gases and hydrogen which are directed to an incinerator. The aqueous solution of HCN is then distilled. Outgoing HCN at the top is condensed at low temperature. The water leaving the bottom of this purification column, after being cooled, is recycled to the absorber. The product obtained at the end of this process has a purity greater than 99% by weight.

These processes, although leading to a product of high purity have the disadvantage of being limited in productivity, because the yields with respect to ammonia generally reach only values of the order of 60-70%.

Different solutions have been sought to increase the yield of the reaction of ammonia on methane. In US Patent 3,102,269 it is proposed to add a small amount of a volatile sulfur-containing compound during the first hours of the synthesis of HCN to reduce the catalyst activation period and increase the selectivity. The preferred compound is carbon disulfide (CS ₂ ), but other sulfur-containing compounds can be used, such as thiophene, mercaptans, such as methyl, ethyl, propyl or butyl mercaptan, thioethers such as dimethyl sulphide or diethyl sulfide, or hydrogen sulphide. The sulfur compound is added at a content equivalent to a sulfur content of between 2 and 200 mg per m ³ of the gaseous reaction mixture, a content lower than 2 mg S / m ³ not giving the desired activation effect, and a larger amount, for example 500 mg, becoming a poison for the catalyst. The yield of HCN increased from 61% to 70% with the addition for 2 hours of the equivalent of 5 mg of sulfur / m ³ gases in CS ₂ . In the article Journal of catalysis, 22, (1971), pages 269-279, it is stated that after a normal production of HCN for a period of about

1000 hours, 100 ppm H ₂ S was added over a period of 110 hours. This had the effect of increasing the yield of HCN by 4% and at the same time the temperature of the platinum-rhodium cloths increased by 20 ^° C.

It is also known that the sulfur that may be present in the methane in large amounts, for example in the form of SO ₂ , H ₂ S, mercaptans or tetrahydrothiophene (odorant), is detrimental to the ammoxidation reaction. Sulfur is also known to modify the phase diagram of the platinum / rhodium alloy by significantly decreasing its melting point and thus modifying the mechanical properties of the catalyst by embrittling it and limiting its lifetime (Massalski, Binary Alloy Diagrams, ASM International, Materials Park Ohio - 1991). According to the article "The Manufacture of Nitric Acid" from the magazine "Platinum Metals Rev., 1967, 11, (2), 60-69" which is analogous to the HCN process, it is strongly recommended to avoid the presence of sulfur compounds in the manufacture of nitric acid, a process which uses, like the HCN process, a reaction between oxygen and ammonia on a rhodium-plated platinum canvas. Such sulfur compounds may for example be found in the lubricants of the compressors used for liquefying ammonia. It is therefore generally recommended to reduce the amount of sulfur compounds in the 5 ppm lubricant. Similarly ammonia, if it is non-synthetic source, may contain significant amounts of sulfur, which must be previously collected, so that their amount does not exceed 1 or 2 ppm. In addition, the air used for the oxidation of ammonia is usually filtered to remove gaseous SO ₂ impurities.

The role of sulfur in the reaction of ammonia with methane appears complex, beneficial under certain conditions, but also harmful under other conditions. The data in the literature on the nature of the sulfur compounds or their quantity to be used are also contradictory.

It has now surprisingly been found that a sulfur compound belonging to the family of polysulfides, added in a small amount to during the production of hydrocyanic acid according to the Andrussow process, or the Degussa process significantly increases the yield of HCN relative to ammonia. This effect is particularly pronounced when the sulfur compound is a disulfide such as dimethyl disulphide. Even minimal progress (ie 1 to 5%) in the yield of the HCN manufacturing process has extremely beneficial consequences in terms of productivity gains. In addition the activity of catalysts decreasing over time depending on the conditions of use, the yield of HCN manufacturing tends to decrease also over time. It is therefore clear the interest of a solution that increases the yield but also to increase the life of the catalyst to improve the profitability of the production unit.

The effect of dimethyl disulphide on the reaction yield was significantly greater than that obtained with H ₂ S usually used in industrial units. Dimethyldisulphide of formula H ₃ CSS-CH ₃ , hereinafter referred to

DMDS, or possibly also called dimethyl disulfide or methyl dithiomethane, is used in a large number of applications. In particular, DMDS is used as a sulphurization or pre-sulphurization agent in refineries in order to activate the hydrotreatment catalysts. DMDS is also used in the petrochemical industry to protect steam cracking systems from coke and carbon monoxide formation. It can also be used as a synthesis intermediate in fine chemistry or metallurgy for its anti-corrosion properties.

Until now, disulfides such as DMDS, or more generally polysulfides, have never been used in a process for producing hydrocyanic acid and its effect is quite unexpected. Without the Applicant being held to any explanation, she thinks that under the operating conditions of the Andrussow process or the Degussa process, the DMDS is broken down into different chemical species which find themselves in equilibrium because of their short residence times in the installation, improving the efficiency of the catalyst, the yield of hydrocyanic acid and reducing the loss of ammonia by decomposition. The object of the present invention is therefore to provide a process for the production of hydrocyanic acid, with improved yield, which makes it possible to reduce the loss of ammonia by decomposition and consequently which leads to a larger production capacity and / or costs. less production. The object of the present invention is also to allow a shorter catalyst activation time, and a longer catalyst life by keeping the same high efficiency longer, thereby improving the cost efficiency of the catalyst. production unit.

It is also an object of the present invention to provide an improved hydrocyanic acid manufacturing process which is simple, fast (with the fewest possible steps), easy to implement, and easily adapts to the production devices of hydrogen cyanide. hydrocyanic acid existing in the industry.

The subject of the present invention is a process for producing hydrocyanic acid in which a gaseous mixture is passed comprising methane (or a natural gas) and ammonia, and optionally air and / or oxygen. on a platinum catalyst, characterized in that at least one sulfur compound corresponding to the general formula (I) is added to the gaseous mixture: R - S - (S) _x - R 'in which R and R', identical or different, represent a linear or branched alkyl or alkenyl radical containing from 1 to 5 carbon atoms, and x is a number ranging from 1 to 5.

According to a preferred embodiment of the invention, a gaseous mixture comprising methane (or a natural gas), ammonia, air and optionally oxygen is passed over a catalyst composed of platinum rhodium. According to another preferred embodiment of the invention, a gaseous mixture comprising methane (or a natural gas) and ammonia is passed through tubes of sintered alumina coated internally with platinum at a temperature of the order 1300 ^° C.

As non-limiting examples of radicals R and R ', mention may be made of methyl, ethyl, propyl, allyl and propenyl radicals. Preferably, the radicals R or R 'are methyl, ethyl or propyl radicals. Among the compounds of formula (I), those for which x is from 1 to 3, from preferably disulphides (x = 1), and more particularly dimethyldisulphide (DMDS).

Dimethyldisulphide (DMDS) is a widely available product, it is in particular marketed by Arkema. The process according to the invention is characterized in that it comprises the addition of a certain amount of sulfur compound corresponding to formula (I). The sulfur compound may be added directly to at least one of the raw materials, methane or natural gas, ammonia, or air or oxygen, upstream of the mixture. The sulfur compound can also be added directly to the gaseous mixture, methane / ammonia, or methane / ammonia / air and / or oxygen, at the mixer or downstream of the mixer in the gas stream before it passes over the catalyst. According to the method of the invention, it is possible to use only one of these possibilities of addition or to combine several of these different possibilities, the sulfur compound may be added by injection at one or more injection points of the process .

The addition of sulfur compound is preferably during the normal course of the reaction although it is also possible to add it during the catalyst activation step (approximately 24 to 48 hours).

The sulfur compound of formula (I) is preferably added continuously to maintain an optimal level of sulfur. The sulfur compound can be added continuously over a period of more than 30 days of operation of the installation.

The amounts of sulfur compound of formula (I) injected into the gaseous mixture range from 5 to 500 ppm expressed by volume of sulfur relative to the volume of methane introduced, preferably from 5 to 200 ppm of sulfur and more particularly from 5 to 100 ppm, and even more preferably from 5 to 50 ppm expressed by volume of sulfur relative to the volume of methane. These amounts of sulfur compound have no adverse impact for the subsequent use of the product obtained from the process according to the invention.

All the other process parameters of the process can be kept constant, compared to the process without the addition of sulfur compound of formula (I). Typically, in an Andrussow process, methane of purity of about 95% is used, the CH ₄ / NH ₃ molar ratio ranges from 1.0 to 1.2, the total molar ratio (CH ₄ + NH ₃ ) / O 2 ranges from 1.5 to 2, preferably 1.6 to 1.9; the pressure is generally 1 to 2 bar; the reaction temperature is between 1050 ^° C. and 1150 ^° C.

Under these conditions of constant constant parameters (purities of raw materials, constant molar ratios, constant temperature and pressure, constant residence time, etc.), the effect of the sulfur compound of formula (I) such as DMDS results in an increase in the yield of HCN of 1 to 5% relative to the ammonia introduced, an increase in the selectivity related to a decrease in the decomposition rate of ammonia and an increase in the catalyst temperature of 10 to 40 ° C . Advantageously, the amount of oxygen introduced if necessary can then be decreased, as well as the amounts of methane and ammonia, which results in an increase in productivity.

The method of the invention makes it possible to dispense with the use of a toxic gas such as H ₂ S and uses a non-toxic liquid product that is easily vaporizable under the conditions of the process (boiling point of about 110.degree. ⁰ C). The sulfur compound of formula (I), such as DMDS, makes it possible to significantly improve the productivity of the catalyst used, without requiring an additional stage of purification of the finished product. Unlike the use of H ₂ S, the product obtained according to the process of the invention is free of sulfur compound such as H ₂ S, which allows its direct use in any subsequent process in which the presence of sulfur is not not desirable, such as a process for preparing acetone cyanohydrin.

Surprisingly, it has been found, moreover, that the sulfur compound of formula (I) such as DMDS has fewer long-term negative effects on the catalyst, particularly from a brittle point of view of the rhodium-plated platinum webs. loss of metal, than other sulfur compounds such as H ₂ S or dimethylsulfide (DMS). The catalysts can therefore be used for a much longer time before being changed. The invention also relates to the use of at least one sulfur compound corresponding to the general formula (I): R - S - (S) _x - R 'in which R and R', which are identical or different, represent a radical alkyl or alkenyl, linear or branched, containing from 1 to 5 carbon atoms, and x is a number ranging from 1 to 5, in an amount effective in a process for producing hydrocyanic acid by reaction of ammonia and methane (or natural gas), to increase the yield of said process.

The product obtained directly from the process according to the invention is advantageously used to produce methionine or the hydroxyanalogue of methionine by reaction with methylmercaptopropionaldehyde (MMP).

Methionine, or 2-amino-4- (methylthio) butyric acid, of chemical formula CH ₃ -S- (CH ₂ ) ₂ -CH (NH ₂ ) -COOH is an essential amino acid, not synthesized by animals, necessary as a supplement in the food ration, especially poultry, whose methionine requirements are important. Methionine obtained by chemical synthesis has been used as a substitute for natural inputs (fishmeal, soya cake, etc.) for animal feed, mainly for poultry.

Unlike other amino acids, methionine is biologically assimilable in both the dextrorotatory (d or +) and laevorotatory (I or -) forms, which has allowed the development of chemical syntheses leading to the racemic product. Thus, the synthetic methionine market is mainly that of dl-methionine, a solid product commonly designated by DLM. There is also a liquid derivative of methionine, α-hydroxy acid, corresponding to 2-hydroxy-4- (methylthio) butyric acid of chemical formula CH ₃ -S- (CH ₂ ) 2 -CH (OH) -COOH, which has the distinction of being converted in vivo to methionine virtually quantitatively. This liquid product, commercially available as 88% by weight aqueous solution, is commonly referred to as hydroxyanalogue of methionine.

Numerous syntheses have been described concerning the methionine or its hydroxylated derivative, but the chemical processes exploited industrially rest essentially on the same main raw materials and the same key intermediates, namely: acrolein and methyl mercaptan (MSH) leading to methylmercaptopropionaldehyde (MMP), also designated by 3- (methylthio) propanal or by methylthiopropionic aldehyde (AMTP),

- Hydrocyanic acid or sodium cyanide (NaCN), which after reaction with MMP finally leads to the methionine or hydroxyanalogue of methionine.

Reference may be made to the article Techniques of the Engineer, Process Engineering Process, J 6-410-1 to 9 which describes the industrial implementation conditions of methionine synthesis processes using methylmercaptopropionaldehyde and the acid hydrogen cyanide as intermediates, one of the processes which can be schematically illustrated by the following reactions:

H ₂ S + CH ₃ OH → CH ₃ SH + H ₂ O

CH ₃ SH + CH ₂ = CH-CHO → CH ₃ -S-CH ₂ -CH ₂ -CHO CH ₃ -S-CH ₂ -CH ₂ -CHO + HCN → CH ₃ -S-CH ₂ -CH ₂ -CH (OH) -CN

CH ₃ -S-CH ₂ -CH ₂ -CH (OH) -CN + NH ₃ → CH ₃ -S-CH ₂ -CH ₂ -CH (NH ₂ ) -CN

CH ₃ -S-CH ₂ -CH ₂ -CH (NH ₂ ) -CN + 2H ₂ O + H ⁺ → CH ₃ -S-CH ₂ ) ₂ -CH (NH ₂ ) -COOH.

Advantageously, the product obtained directly from the process according to the invention is also used to produce acetone cyanohydrin by reaction with acetone according to the reaction:

CH ₃ -C (O) -CH ₃ + HCN → (CH ₃ ) ₂ C (OH) CN

Acetone cyanohydrin is an intermediate compound for producing methyl methacrylate (MMA) according to the two routes schematized hereinafter.

A first route is to form α-oxyisobutyramide monosulfate, which is converted into sulfuric methacrylamide. The latter is then hydrolyzed and esterified with methanol to form methyl methacrylate.

A second way is to react directly methanol, then to implement a dehydration reaction to lead to methyl methacrylate. One can refer to the article Techniques of the Engineer, Process Engineering Process, J 6-400-1 to 6 which describes the conditions of implementation process of production of methyl methacrylate according to the cyanohydrin pathway of acetone.

NH ₂ .HSO ₄

Advantageously, the product obtained directly from the process according to the invention is also used to produce adiponitrile by reaction with butadiene according to the reaction:

CH ₂ = CH-CH = CH ₂ + 2 HCN → NC- (CH ₂ ) ₄ -CN Adiponitrile, after hydrogenation, leads to hexamethylenediamine, which is an intermediate compound to produce polyamide 6-6 (Nylon ^® ) by polycondensation of hexamethylenediamine adipate. Reference may be made to the article Techniques of the Engineer, Process Engineering Processes, J 6-515-1 to 7 which describes the synthesis of polyamide 6-6 along this route.

Advantageously, the product obtained directly from the process according to the invention is also used to produce sodium cyanide by neutralization with sodium hydroxide according to the reaction:

HCN + Na OH → Na CN + H ₂ O Sodium cyanide has many applications, particularly for the extraction of precious metals, electroplating or the synthesis of chemical compounds. The following examples illustrate the present invention without limiting its scope.

EXAMPLES Example 1

Natural gas (GN), without sulfur compound and 95% methane volume titrate, is mixed with ammonia, air and oxygen in CH ₄ / NH ₃ volume proportions of 1, 16 and ( CH ₄ + NH ₃ ) / O ₂ total of 1.70. The GN flow rate is 4000 kg / h. The mixture is sent through a bed of 15 Pt / Rh webs (90/10). The yield of HCN relative to ammonia stabilizes at 68.0% after 48 hours, the temperature is then about 1060 ^° C. DMDS is added at a rate of 10 ppm expressed by volume of sulfur in relation to the volume of methane. Very quickly, the yield increases to 70.0%, the decomposition rate of ammonia, determined from the analysis of the gases N ₂ and H ₂ at the exit, falls by 2%, and the temperature of the fabrics increases by + 10 ⁰ C. The DMDS is injected continuously which allows to maintain performance for more than 60 days. Stopping the injection of DMDS causes a gradual drop in yield. The pure HCN product does not contain any sulfur compound.

Example 2

The same protocol as Example 1 is reproduced except that the DMDS is added at a rate of 125 ppm expressed in volume of sulfur relative to the volume of methane. Very quickly, the yield increases to 73.0%, the decomposition rate of ammonia falls by 4% and the temperature of the fabrics increases by + 40 ° C. The DMDS is injected continuously which allows to maintain the performance for more than 50 days. The pure HCN product does not contain any sulfur compound.

Example 3 (comparative)

The same protocol as Example 1 is reproduced except that I ¹ H ₂ S is added in place of the DMDS. L ¹ H ₂ S is added at a rate of 100 ppm expressed by volume The yield of HCN increases by only 1.0%, the decomposition rate of the ammonia and the temperature of the fabrics remain practically unchanged. In addition, the pure HCN produced contains traces of H ₂ S harmful to the downstream application.

Example 4

Natural gas (GN), without sulfur compound and 95% methane volume titrate, is mixed with ammonia, air and oxygen in CH ₄ / NH ₃ volume proportions of 1, 16 and ( CH ₄ + NH ₃ ) / O ₂ total of 1.73. The GN flow rate is 4100 kg / h. The mixture is sent through a bed of 18 canvases Pt / Rh (90/10). The yield of HCN with respect to ammonia is 67% after 50 days of run, the temperature is then about 1060 ^° C. DMDS is added at a rate of 15 ppm expressed by volume of sulfur relative to the volume of methane. Very quickly, the yield increases to 69%, the decomposition rate of ammonia, falls by 3.5%, and the temperature of the fabrics increases by + 10 ^° C.

Stopping the injection of DMDS causes the yield to drop to 67%, the ammonia decomposition rate to increase by 3.5% and the temperature drop to 10 ° C. The DMDS is injected again at a rate of 25 ppm expressed as a volume of sulfur relative to the volume of methane. Very quickly, the yield increases to 69%, the decomposition rate of ammonia falls by 3.5%, and the temperature of the fabrics increases by 10 ⁰ C.

Example 5

In a reactor other than that of the preceding examples, natural gas (GN), without sulfur compound and titrating 97% volume in methane, is mixed with ammonia, air and oxygen in volume proportions CH ₄ / NH ₃ of 1.09 and (CH ₄ + NH ₃ ) / O ₂ total of 1.95. The GN flow rate is 3120 kg / h. The mixture is sent through a bed of 20 Pt / Rh webs (90/10). The yield of HCN relative to ammonia stabilizes at 64.0% after one week of operation. DMDS is added at a rate of 10 ppm expressed as a volume of sulfur relative to the volume of methane. Very quickly, the yield increases to 67.0%, the decomposition rate of ammonia falls, the temperature of the cloths increases. The DMDS is injected continuously, which allows, despite ammonia and natural gas flow variations, to keep the yield at least 67% for 60 days. A 68% yield is even obtained by increasing the DMDS concentration to 20 ppm expressed by volume of sulfur relative to the volume of methane. Stopping the injection of DMDS causes a drop in yield.

Example 6

Natural gas (GN), without sulfur compound and titrating 94% volume in methane, doped with DMDS at a rate of 20 ppm expressed in volume of sulfur with respect to the volume of methane, is mixed with ammonia, air and oxygen in volume proportions CH ₄ / NH ₃ of 1, 15 and (CH ₄ + NH ₃ ) / O ₂ total of 1.76. The GN flow rate is 3840 kg / h. The mixture is sent through a bed of 20 Pt / Rh webs (90/10). The yield of HCN with respect to ammonia is 66.5%, the temperature is then about 1060 ^° C. Stopping the injection of the DMDS causes a fall in the yield to 64%, an increase in the decomposition of ammonia 3.5% and a temperature drop of 10 ⁰ C.

L ¹ H ₂ S is then added at a rate of 3 ppm expressed as a volume of sulfur relative to the volume of methane. The yield of HCN increases by less than 1.0%, the decomposition rate of the ammonia and the temperature of the fabrics remain virtually unchanged.

Stopping the injection of H ₂ S causes a small drop in efficiency to 64%.

Claims

A process for producing hydrocyanic acid in which a gaseous mixture comprising methane (or a natural gas) and ammonia, and optionally air and / or oxygen, is passed over a catalyst. platinum, characterized in that at least one sulfur compound corresponding to the general formula (I) R - S - (S) _x - R 'is added to the gaseous mixture, in which R and R', which may be identical or different, represent a linear or branched alkyl or alkenyl radical containing from 1 to 5 carbon atoms, and x is a number ranging from 1 to 5, preferably from 1 to 3.

2. Method according to claim 1 characterized in that the radicals R and R 'are chosen from methyl, ethyl or propyl radicals.

3. Method according to claim 1 or 2 characterized in that the sulfur compound is a disulfide, preferably dimethyldisulphide.

4. Process according to any one of the preceding claims, characterized in that the sulfur compound is added directly to at least one of the raw materials, methane or natural gas, ammonia, or air or oxygen, upstream of the mixture.

5. Method according to any one of claims 1 to 3 characterized in that the sulfur compound is added directly into the gas mixture at the mixer.

6. Method according to any one of claims 1 to 3 characterized in that the sulfur compound is added to the gaseous mixture downstream of the mixer in the gas stream before passing over the catalyst.

7. Process according to any one of the preceding claims, characterized in that the amount of sulfur compound added ranges from 5 to 500 ppm. expressed in volume of sulfur relative to the volume of methane, preferably from 5 to 100 ppm by volume of sulfur relative to the volume of methane.

8. Process according to any one of the preceding claims, characterized in that the sulfur compound is injected continuously.

9. Use of at least one sulfur compound corresponding to the general formula (I) R - S - (S) _x - R 'in which R and R', which may be identical or different, represent a linear or branched alkyl or alkenyl radical , containing 1 to 5 carbon atoms, and x is a number from 1 to 5, in an amount effective in a process for producing hydrocyanic acid by reaction of ammonia and methane (or natural gas), to increase the yield of said process.

Use of the product obtained directly from the process of any one of claims 1 to 9 to produce methionine or the hydroxyanalogue of methionine by reaction with methylmercaptopropionaldehyde.

Use of the product obtained directly from the process of any one of claims 1 to 9 to produce acetone cyanohydrin by reaction with acetone.

12. Use of the product obtained directly from the process according to any one of claims 1 to 9 to produce adiponitrile by reaction with butadiene.

Use of the product obtained directly from the process of any one of claims 1 to 9 to produce sodium cyanide by neutralization with sodium hydroxide.